Heat exchanger of porous metal



March 18, 1969 R. B. FLEMING 3,433,299

HEAT EXCHANGER 0F POROUS METAL Filed Feb. 16. 1967 MARX - [/7 ver; 2: or. Robert B. F/em/h g,

HAS A i: or'ney United States atent 3,433,299 HEAT EXCHANGER OF POROUS METAL Robert B. Fleming, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Feb. 16, 1967, Ser. No. 616,644 U.S. Cl. 165135 3 Claims Int. Cl. F28f 1/10 ABSTRACT F THE DISCLOSURE A heat exchanger made of two or more channels in which fluids pass in counterflow or parallel flow through a plurality of plates made of porous heat conductive material. The plates extend across the channels and are in thermal contact from one channel to the adjacent channel or channels so that heat may be conducted laterally while the fluids are flowing longitudinally in counterflow or parallel flow relation.

My invention relates principall to a cryogenic heat exchanger and particularly to a cryogenic heat exchanger utilizing porous metal as the heat transfer medium.

Prior art heat exchangers have utilized thin walls or extended surfaces through which heat has been passed. These thin walls are found in a variety of shapes such as tubes, coils, plates or fins. A defect of prior heat exchangers, particularly exchangers of very high thermal effectiveness such as are found in cryogenic service, is that a large amount of heat is transferred longitudinally, i.e. along the same direction as the streams, by the conductive wall which separates the streams of flowing material. Thus heat is passed from the warm end of the exchanger to the cold end, rather than from one stream to another. This longitudinal heat transfer greatly reduces thermal performance of high effectiveness counterflow heat exchangers.

Another disadvantage of most prior heat exchangers is the relatively large volume which such heat exchangers must occupy in order that the walls are of suflicient area that the fluid streams will be exposed to the separating walls sufficiently to effect heat transfer between fluid streams.

Another disadvantage of many prior heat exchangers is that their performance suffers from non-uniform flow distribution of the fluid streams. This effect is particularly true in tubular exchangers where the tubes extend the entire length of the exchangers; lower or higher flow in a tube thus affects the efficiency of that tube.

It is an object of my invention to provide a very large heat transfer surface per unit of heat exchanger volume.

It is another object of my invention to provide higher thermal resistance to the transfer of heat longitudinally, in the direction of flow of the fluids.

It is another object of my invention that a very uniform flow distribution exist at any cross-section throughout the heat exchanger and/or, if non-uniformities do occur as a result, for example, of plugging of some of the passages, then the flow non-uniformity will be only local in nature and not affect the performance of the entire heat exchanger to a significant extent.

It is a principal object of my invention to provide maximal heat transfer across the path of flow of the fluids and minimal heat transfer longitudinally through the body of the heat exchanger in the direction of flow of the fluids.

My heat exchanger, in brief, has channels located one adjacent to the other with a thin wall of material having low heat conductivity separating one channel from the other. Across each channel are separate porous members made of a material having high thermal conductivity. A

gap exists between each porous member in the direction of flow, and each member is integral with or located adjacent to another member in another channel. In this way, fluid passes through the porous member and gains or loses heat as it passes through. Heat then passes transversely through the porous member into the adjacent porous member and thence into the fluid in the other channel. Thus, a maximum amount of heat is transferred from one channel to the next through the porous members and a minimum amount of heat is transferred longitudinally i.e.

along the axis of the heat exchanger.

The novel features which are believed to be characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 illustrates a perspective view of the rectangular heat exchanger.

FIGURE 2 shows a perspective cutaway view of the rectangular heat exchanger partially in section.

FIGURE 3 shows a cross-section of a cylindrical heat exchanger.

FIGURE 4 shows two porous plates of the type used in the cylindrical heat exchanger.

The rectangular heat exchanger 1 of FIGURE 1 i shown here in perspective and the arrows indicate the direction of fluid flow. It is noted that the arrows in the top section 2 indicate fluid flow in one direction whereas the arrows in the bottom section 3 indicate fluid flow in a counter direction. In this way, a maximum of heat may be transferred from one fluid to another. If the channel 2 above is smaller in dimension than the channel 3 below, then the density or rate of flow of the fluids may be dif ferent and maximum transfer still be elfected. The channel above, as shown, is a high pressure gas whereas the channel below is larger and adapted for flow of a low pressure gas.

If one wishes a low pressure drop across the porous member then the face area of the porous member is increased.

An essential characteristic of my apparatus is that the thermal conduction in my apparatus is high across the direction of flow but low along the direction of flow.'

This is accomplished in the embodiment shown in FIG- URES l and 2. As best shown in FIGURE 2 there are spaces 4 between the porous members 5 or plates, and the separating wall 6 is made of a thin material of low thermal conductivity, such as thin stainless steel attached or bonded to the porous metal plates of a highly conductive material such as aluminum or copper. The cutaway section of the header 7 and the stripped side shows the porous members 5 and the spaces 4 between the members.

For example, one method of accomplishing the bond between member and wall is by having strips of conductive material such as aluminum attached to the stainless steel wall 6 and to these strips are brazed or otherwise attached the porous metal member. Copper may be favored material for a porous metal member since copper is sinterable or aluminum may be favored for its light Weight. One advantage of my construction is that the fluid passing through porous high thermal conduction plates 5 is forced into interrupted flo-w from plate to plate through the series thus breaking up the thermal boundary layers to give high heat transfer coeflicients. Further, there is high thermal conduction laterally from one stream to another while there is very little conduction down the thin stainless steel separating wall 6. Any number of channels may be mounted one above the other without departing from the spirit of my invention; in this case, more than two fluid streams could be accommodated.

The porosity of the porous members may be obtained in a number of ways. For example, the members may consist of sintered metal spheres, or sintered layers of wire screen, or sintered randomly oriented small metal pieces. Alternatively, the members may consist of plates into which multiple holes are produced by a process such as drilling, punching, etching, etc. In the latter case the holes may be arranged so that the holes will. not be aligned from plate to plate in the assembly. If the holes are aligned for some reason, the plates are spaced a distance apart suflicient to permit fluid mixing and the formation of new thermal boundary layers in each plate as the stream passes through.

Whichever method is used to produce the members, if the resulting passages are small enough and numerous enough, then a very large heat transfer surface area per unit volume will result. Further, the irregular path the fluid is forced to follow prevents the buildup of hydrodynamic and thermal boundary layers with each succeeding plate because of interruption of flow. It is well known in heat transfer practice that considerably higher heat transfer coefiicients occur where new thermal boundary layers are formed.

Further, in the space after each porous member the fluid enters a constant pressure zone which provides a more uniform flow distribution through each cross-section of the heat exchanger than is usually the case in heat exchangers where the flow is not split up and recombined a number of times in the course of passage through the exchangers. Conventional heat exchangers frequently have poor performance because of non-uniform flow distribution.

The combination of high heat transfer coeflicient and large heat transfer surface area gives a compact, efficient heat transfer apparatus.

Another embodiment of this invention is shown in FIG- URE 3 wherein fluid passes in one direction through the center channel member 8 of the cylindrical heat exchanger 9 and in another direction through the annular channel member 10 outside of the first channel. Obviously, any number of annular channels may be layered without departing from the spirit of my invention.

In the embodiment of FIGURE 3 which in principles is much like the embodiment of FIGURE 1, heat is passed laterally through the thin low thermal conducting wall of the porous plates but is not passed longitudinally down the axis of the heat exchanger. As in the embodiment of FIGURE 1 of my invention, the wall material of low heat conductivity may be stainless steel and the plates of high heat conductivity may be aluminum or copper. The same principles apply to the cylindrical heat exchanger as to the rectangular heat exchanger. The porous plates of FIGURE 3 are shown in FIGURE 4 in more detail. The inner plate 11 lies within channel member 8 and outer plate 12 is mounted inside channel member 10 and surrounds channel member 8. These plates are attached to channel member 8 in such a fashion as to readily pass heat through it.

The foregoing is a description of an illustrative embodiment of the invention, and it is applicants intention in the appended claims to cover all forms which fall within the scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A heat exchanger for transferring heat between two counter flowing fluid streams comprising a plurality of longitudinally continuous, parallel low thermal conductivity metal members forming two adjacent channels for such streams and separating one stream from the other, each of said channels having a plurality of spaced, substantially parallel high-conductivity porous members therein extending between the walls of the respective channels, each of said porous members being brazed to the metal members forming the channel in which it is contained, adjacent porous members in each channel being spaced apart by a distance which is large relative to the thickness of the metal member separating the channels, said porous members having a longitudinal thickness which is large relative to the thickness of said separating metal member, and each porous member in one channel being substantially coplanar with an adjacent porous member in the other channel whereby the conduction of heat through said separating metal member from a porous member in one channel to an adjacent porous member in the other channel is facilitated and conduction of heat longitudinally in each channel is inhibited by the low thermal conductivity path between adjacent porous members in each channel.

2-. A heat exchanger of claim 1 wherein one of the channels is a cylindrical inner channel and the other is an annular outer channel.

3. The heat exchanger of claim 1 in which both of the channels are rectangular in cross section.

References Cited UNITED STATES PATENTS 1,863,586 6/1932 Wilke 135 2,448,315 8/1948 Kunzog 165-154 XR 3,228,460 1/1966 Garwin 165-81 3,364,991 l/1968 Wang 165--135 FOREIGN PATENTS 147,857 1/ 1904 Germany. 147,858 1/1904 Germany.

FRED C. MATT-ERN, JR., Primary Examiner.

MANUEL ANTONAKAS, Assistant Examiner.

US. Cl. X.R. 

